The present invention relates to data communications equipment and, more particularly, to overload protection circuitry on data interface leads.
Data communications equipment (DCE) interface to other peripheral equipment via data interface, or interchange, circuits, which are typically governed by industry standards. These interface circuits typically comprise a number of interface signals that are received by, and supplied from, the DCE. For example, in a DCE such as a Digital Service Unit (DSU), various industry standards such as RS-530, RS-449, and V.35, specify each interface signal according to function, pin placement, and electrical characteristics like operating voltage range, etc.
In the design of a DCE, commercially available integrated circuits are typically used to receive and supply the various interface signals. When supplying the interface signals, the DCE uses a "driver" integrated circuit (driver IC) to generate the output signals. The driver IC is designed by the respective integrated circuit manufacturer to conform to the specified electrical characteristics of a particular industry standard like those mentioned above. The output signals can either be differential, or single-ended, and appear on a number of output pins, or leads, of the DCE, which can then be coupled to an external peripheral either via a cable or via a backplane. However, when an interconnection is being made between a piece of peripheral equipment and a DCE, the danger always exists that a "short" may occur on one, or more, of the output pins. For example, there can be an external short either to ground or to the opposite leg of a differential output. Further, the driver IC includes a current limiter that typically passes more current than the power supply of the DCE is designed to handle. As a result, if a short occurs on one, or more, of these output leads, the potential exists for overloading--and damaging--the power supply of the DCE.
Unfortunately, simple resistive current-limiting cannot be used to protect the power supply of the DCE due to loaded versus unloaded output voltage constraints that are imposed by the above-mentioned standards on output signals. For example, insertion of a simple resistor in the output signal path, i.e., between a driver IC output pin and a load, sets up a voltage divider between this resistor and the load. Although this resistor may limit the current flow when a short occurs on the output pin of the driver IC, during normal operation the voltage drop across this resistor skews the operating voltage range of any output signal generated by the driver IC in such a way as to violate industry standards.
Lacking a simple solution, some manufacturers provide a power source that has a power rating higher than nominally required to take into account that one, or perhaps more, external leads will be shorted over the course of operation. This increase in power allows the power supply of the DCE to either supply more current than is required during normal operation, or to provide a significantly higher voltage than is required by the driver IC. In the latter case, instead of using a +5 volt power supply, a +10 volt power supply is used to provide power. The +10 volts is then regulated down by a series resistor and an integrated circuit voltage regulator, which supplies +5 volts to the driver IC. This configuration allows a significant voltage drop to occur across the series resistor, yet still provides enough input voltage to the voltage regulator, which then provides the required +5 V to the driver IC. However, both these approaches add cost to the design of the DCE. To avoid this additional cost, other manufacturers may simply not guarantee operation of their equipment if an external short should ever occur.